In Texas, you need what is called a "Guard Card".
Just like you need a license to drive a car, you need a license to be a security guard. Yes, even the "rent-a-cop" kind. Just like there are different types of drivers license if you want to drive trucks, buses, cars, or motorcycles, the same concept applies for being a security guard. For example, if you want to be an unarmed security card you need one type of license. If you want to do armed security you need another type.
I am not sure if you have to be hired by a company first before you can get your license but I know the process involves filling out an application with the Texas Department of Public Safety's "Private Security Bureau", getting finger-printed, getting a criminal background check done, attending training, and paying an application fee. It usually takes about 4 to 6 weeks to get everything processed and approved before you get your "Guard Card".
Here is a link to the Texas d*s website:
I would call the Private Security Bureau and ask them what the process for getting hired as a security guard is. Here is the phone number to their office in Austin: (512) 424-2000
Good Luck. I hope this helps!!